This invention relates to making thin strip and more particularly casting of thin strip by a twin roll caster.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotating horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or tundish/distributor, from which it flows through a metal delivery nozzle located above the nip, which directs the molten metal to form a casting pool supported on the casting surfaces of the rolls above the nip. This casting pool is typically confined at the ends of the casting rolls by side plates or dams held in sliding engagement adjacent the ends of the casting rolls.
In casting thin strip by twin roll casting, the metal delivery nozzles receive molten metal from the movable tundish and deposit the molten metal in the casting pool in a desired flow pattern. Previously, various designs have been proposed for delivery nozzles involving a lower portion submerged in the casting pool during a casting campaign, and having side openings through which the molten metal is capable of flowing laterally into the casting pool outwardly toward the casting surfaces of the rolls. Examples of such metal delivery nozzles are disclosed in Japanese Patent No. 09-103855 and U.S. Pat. No. 6,012,508. In prior art metal delivery nozzles, there has been a tendency to produce thin cast strip that contains surface defects and associated microcracking from uneven solidification at the chilled casting surfaces of the rolls.
The present invention provides an apparatus and method for continuous thin strip casting that is capable of substantially reducing and inhibiting such surface defects and microcracks in the cast strip, and at the same time reducing wear in the delivery nozzles and the costs in thin strip casting. By testing, we have found that a major cause of such defects is premature solidification of molten metal in the regions where the casting pool meets the casting surfaces of the rolls, generally known as the “meniscus” or “meniscus regions” of the casting pool. In these regions, if solidification occurs before the molten metal has made contact with the roll surface, irregular initial heat transfer can occur between the metal shell and the casting rolls, resulting in formation of surface defects, such as depressions, ripple marks, cold shuts and/or microcracks. The temperature of the metal in the surface region of the casting pool between the rolls tends to be lower than that in the incoming molten metal. If the temperature of the molten metal at the pool surface in the region of the meniscus becomes too low then surface cracks and “meniscus marks” (i.e., marks on the strip caused by the meniscus freezing while the pool level is uneven) are likely to occur
One way of dealing with such surface cracks and meniscus marks has been to increase the temperature of the incoming molten metal from the delivery nozzle, so that molten metal reaches the casting surfaces of the casting rolls before reaching solidification temperatures. Another approach has been to cause the incoming molten metal to be delivered relatively rapidly into the meniscus regions of the casting pool directly from the delivery nozzle. This reduced the tendency for premature solidification of the metal before it contacts the casting roll surfaces. This approach has been more effective in reducing surface defects in the cast strip. Examples of this approach are to be seen in Australian Patent Application 60773/96. This approach has allowed for casting of thin strip with reduced formation of surface defects and cracks.
Nevertheless, the formation of pieces of solid metal known as “skulls” in the casting pool in the vicinity of the confining side plates or dams have been observed. The rate of heat loss from the casting pool is higher near the side dams (called the “triple point region”) due to conductive heat transfer through the side dams to the casting roll ends. This localized heat loss near the side dams has a tendency to form “skulls” of solid metal in that region, which can grow to a considerable size and fall between the casting rolls and causing defects in the cast strip. An increased flow of molten metal to these “triple point” regions, the regions near the side dams, have been provided by separate direct flows of molten metal to these triple point regions. Examples of such proposals may be seen in U.S. Pat. No. 4,694,887 and in U.S. Pat. No. 5,221,511. Increased heat input to these triple point regions has inhibited formation of skulls.
Australian Patent Application 60773/96 discloses a method and apparatus in which molten metal is delivered to the delivery nozzle in a trough closed at the bottom. Side openings are provided through which the molten metal flows laterally from the nozzle into a casting pool in the vicinity of the casting pool surface. The flow of molten metal into the casting pool was improved; however, unevenness in metal flow adjacent the casting roll surfaces still tended to cause surface defects and surface cracks in the cast strip. Further, there remained concern for wear on the delivery nozzle caused by the impact of the molten metal due to ferrostatic pressure, and turbulence caused as the molten metal moved through the delivery nozzle to discharge laterally into the casting pool below the meniscus of the casting pool. In addition, there was concern for extending the useful life of the delivery nozzles and in turn reducing the cost of producing thin cast strip.
The present invention provides an improved apparatus for casting metal strip and method of continuously casting metal strip. Disclosed is an apparatus for casting metal strip comprising:                (a) a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip therebetween through which cast strip can be cast, and on which a casting pool of molten metal can be formed supported on the casting surfaces above the nip with side dams adjacent ends at casting rolls to confine the casting pool,        (b) a delivery nozzle disposed above the nip capable of delivering molten metal to form the casting pool supported on the casting rolls, the delivery nozzle comprising:                    segments each having an elongate nozzle body with longitudinally extending side walls, end walls and a bottom part to form an inner trough,            a nozzle insert disposed above bottom portions of the inner trough of each segment and supported relative to the nozzle body and through which incoming molten metal may be delivered to the inner trough of each segment of the delivery nozzle, and            the elongate nozzle body of each segment having passageways in fluid communication with the inner trough and outlet openings capable of discharging molten metal from the nozzle body outwardly into the casting pool.                        (c) a distributor capable of supplying molten metal to form the casting pool through the nozzle insert, inner trough and passageways of the segments of the delivery nozzle.        
The nozzle insert may be positioned to at least partially protect inlets of passageways of the delivery nozzle from the impact of incoming molten metal from the metal distributor. In addition, the nozzle insert may include an end wall acting as a weir to separate the flow of molten metal between the inner trough and a nozzle end portion, where the nozzle end portion is capable of supplying molten metal adjacent an end portion of the casting surfaces of the casting rolls. Further, the nozzle insert may include side walls each having a transition surface from a substantially horizontal orientation to a substantially vertical orientation.
The nozzle insert may also be capable of being placed in the relation to the segments of the delivery nozzle as desired. Also, the nozzle insert may be funnel shaped and extend above the nozzle body of each segment of the delivery nozzle, to further inhibit streaming molten metal from the metal distributor bypassing the inner trough of the segments of the deliver nozzle and entering the casting pool.
Each nozzle insert may be supported relative to the nozzle body by the side walls. Alternatively or in addition, at least one support member may be disposed between the nozzle insert and a bottom portion of the inner trough for supporting the nozzle insert spaced from the bottom portion.
Each segment may be assembled with opposing side walls and an inner trough extending along the side walls to form a shoulder portion between the side walls and the inner trough, and with a plurality of holes extending through each shoulder portion and communicating with side outlets adjacent bottom portions of the segments of the delivery nozzle. By this arrangement, molten metal is capable of flowing into the inner trough, from the inner trough through the holes between the inner trough and sidewalls, and exit the delivery nozzle through the side outlets in a substantially lateral direction into a casting pool. In this embodiment, the bottom of the inner trough may be co-extensive with the entry to the holes into the shoulder portion, and the outlet openings may be spaced longitudinally along the side walls adjacent the bottom part. Each segment may be made in one integral refractory piece, and the nozzle insert may be made in one or more refractory pieces fitted into each segment of the delivery nozzle.
Alternatively, each segment of the metal delivery nozzle may be assembled with at least one partition extending between the side walls, and with the passages between the inner trough and side walls extending between the partitions or between a partition and end wall. In this embodiment, the nozzle inserts may be separately positioned between partitions and end walls portioned by the side walls, or formed as one unit to be positioned above partitions between end walls or formed between and/or over partitions. If desired, in this embodiment, each segment may be made in one integral refractory piece, and the nozzle insert made in one or more refractory pieces fitted into each segment of the delivery nozzle.
In still another embodiment, each segment of the metal delivery nozzle may be assembled with the inner trough and side walls in separate pieces, pinned together with ceramic pins. Protrusions may extend into the passages from the inner trough or side wall, or both, to direct the molten metal flowing through the passages. The protrusions may be in rows aligned or offset as desired to provide the desire flow characteristics to the molten metal as it flows through the passages from the inner trough to the outlet openings adjacent the bottom part of the segment of the delivery nozzle.
Also disclosed is a method of continuously casting metal strip comprised of the steps of:                (a) assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip therebetween through which cast strip can be cast,        (b) assembling segments of a delivery nozzle above the nip with each segment having an elongate nozzle body including longitudinally extending side walls, end walls and a bottom part to form a nozzle trough in the nozzle body and having passageways in fluid communication between the nozzle trough and outlet opening capable of discharging molten metal from the nozzle body laterally into the casting pool,        (c) assembling a nozzle insert above the trough of each segment of the delivery nozzle to withstanding a part of the impact of the incoming molten metal and at least partially protecting the passageways of the delivery nozzle from the impact of incoming molten metal to the segments of the delivery nozzle,        (d) forming a casting pool of molten metal on the casting surfaces above the nip, and        (e) delivering molten metal from a metal distributor through the nozzle insert, and the nozzle trough and passageways of the delivery nozzle to discharge molten metal laterally into the casting pool.        
In this method of continuously casting metal strip, each segment may be assembled with opposing side walls and an inner trough extending along the side walls to form a shoulder portion between the side walls and the inner trough, and with a plurality of holes extending through each shoulder portion and communicating with side outlets adjacent bottom portions of the segments of the delivery nozzle, such that molten metal is capable of flowing into the inner trough, from the inner trough through the holes between the inner trough and sidewalls, and exit the delivery nozzle through the side outlets in a substantially lateral direction into a casting pool. In this embodiment, the bottom of the inner trough is co-extensive with the entry to the holes into the shoulder portion, and the outlet openings may be spaced longitudinally along the side walls adjacent the bottom part. Each segment may be made in one integral refractory piece, and the nozzle insert made in one refractory piece may be fitted into each segment of the delivery nozzle.
Alternatively, in the method of continuously casting metal strip, each segment of the metal delivery nozzle may be assembled with at least one partition extending between the side walls, and with the passages between the inner trough and side walls extending between the partitions or between a partition and end wall. In this embodiment, the nozzle inserts may be separately positioned between partitions and end walls portioned by the side walls, or formed as one unit to be positioned above partitions between end walls or formed over partitions. If desired, in this embodiment, each segment may be made in one integral refractory piece, and the nozzle insert made in one or more refractory pieces fitted into each segment of the delivery nozzle.
In still another embodiment of the method of continuously casting metal strip, each segment of the metal delivery nozzle may be assembled with the inner trough and side walls in separate pieces, pinned together with ceramic pins. Protrusions may extend into the passages from the inner trough or side wall, or both, to direct the molten metal flowing through the passages. The protrusions may be in rows aligned or offset as desired to provide the desire flow characteristics to the molten metal as it flows through the passages from the inner trough to the outlet openings adjacent the bottom part of the segment of the delivery nozzle.
In each embodiment of both the improved delivery nozzle and method of casting steel strip with the delivery nozzle, each segment of the delivery nozzle is provided with a nozzle insert that assists in absorbing the kinetic energy of the molten metal entering the inner trough of the segments of the delivery nozzle, assists in protecting the passageways of the segments of the delivery nozzle from the impact of the molten metal, inhibits splashing and reduces turbulence in the flow of molten metal through the nozzle, and is relatively inexpensive and can be easily replaced. In addition, the inner trough dissipates a substantial part of the kinetic energy present in the molten metal by reason of downward movement through the metal delivery system from the tundish to the metal distributor to the delivery nozzle. The combination of the nozzle insert and inner trough through the passages to the side outlets further reduces the kinetic energy in the molten metal before reaching the casting pool. As a result, a more uniform and more quiescent flow of molten metal is provided to the casting pool to enhance uniform formation of the cast strip.
The nozzle insert also may be replaceable to further extend the useful life of the delivery nozzle.
Various aspects of the invention will be apparent from the following detailed description, drawings and claims.